Apparatus and method to control the temperature of a melt stream

ABSTRACT

Disclosed is an apparatus for controlling the temperature of a melt stream, a method of using the apparatus to make variable-density polymeric articles, and the polymeric article formed thereby. The apparatus for making the article includes a row of baffles wherein the baffles include a temperature-control conduit defined or disposed within at least one of the baffles. A polymer melt is passed through the apparatus while the temperature of the baffles is varied (which has the effect of altering the density of certain polymer melts). The process can be used to yield a continuous, monolithic polymeric article of variable density. The apparatus can also be used to control the temperature of the material passing through the apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

Priority is hereby claimed to provisional application Ser. No.60/700,294, filed Jul. 19, 2005, incorporated herein.

FIELD OF THE INVENTION

The invention is directed to an apparatus that can be used as a staticmixer head and/or a temperature regulator in processes requiringtemperature control, such as when heating or cooling a melt stream, orother flowing material. The invention is also directed to a continuousmethod for making composite articles of manufacture, such as compositecores for structural and insulating applications, and the articles ofmanufacture produced using the method.

BACKGROUND

The transportation, construction, and manufacturing industries are inconstant need for a cost-effective composite core material with goodmechanical properties, low weight, and efficient insulatingcapabilities. For instance, composite structural panels to be used invehicles and in constructing commercial and residential structuresshould have good thermal-insulating, sound-dampening, and/orshock-absorbing characteristics. For these applications it is common touse sandwich composites having a foamed polymeric core.

Composite sandwiches are commonly used as structural panels inapplications where a stiff and lightweight construction is required.FIG. 1 shows the cross-section of a conventional, prior art compositesandwich composed of one middle core and two outer skins. The cores andthe skins are fabricated separately and then joined in face-to-faceorientation. The resulting sandwich construction is stiffer, lessexpensive and lighter than an equivalent solid panel. The sandwichconstruction takes advantage of the fact that the bending stiffness ofthe resulting panel is proportional to the square of the distancebetween skins. The relatively low-weight core separates the skins to thegreatest extent practicable, which dramatically increases the bendingstiffness of the panel, while adding very little additional weight tothe panel. Further, when a low-stiffness core is used, such aslow-density foam, the core does not directly contribute to the stiffnessof the panel. Although a stiffer core can contribute to the bendingstiffness of the panel, the main advantage of using a stiffer core is toincrease the compression and shear stiffness of the core. Depending onthe application, the core also improves the thermal insulating,impact-cushioning, and sound-dampening qualities of the panel. Thenature of the core can also help to prevent buckling and wrinkling ofthe panel. While less expensive than a solid panel, a sandwich-typeconstruction requires a relatively complex series of manufacturing stepsto join the skins with the core.

Currently, extruded polymeric composite cores are produced from foamedpolymers or from non-foamed polymers with internal reinforcing profiles.Foamed sheets are very commonly used as core materials [1-7].

Extruded high-density structural foams possess desirable mechanicalproperties; however, they are very heavy. Conversely, softer,low-density foams with higher foaming ratios have lower mechanicalproperties, yet they are relatively lightweight. Both foam cores aregood thermal insulators, but softer foams are better sound insulatorsand have better load absorption capabilities. Further, extrudednon-foamed polymeric cores offer good mechanical properties whileremaining lightweight. These cores include internal reinforcementprofiles in order to increase stiffness, while maintaining the remainderof the core hollow [1, 7-8]. However, the hollow cavities offer poorthermal insulating capabilities and provide no structural stiffening tothe core. Therefore, a secondary process is often required to fill thehollow cores with low-density polymeric foam. The foam acts as a thermalinsulator while simultaneously increasing the compressive and impactmechanical properties of the core. Unfortunately, these reinforced foamsare very expensive because of the complicated multi-step manufacturingprocess required to make them.

Other lightweight composite panels are known. For example, compositepanels made of plastic, paper and metal have good mechanical andinsulating properties [9-12]. However, these types of panels typicallyrequire more than one secondary process during their fabrication, suchas injecting, cutting, adhering, or other secondary assembling steps.The more secondary processes necessary for manufacture, the greater thecost of the final structural insulating core. In contrast, conventionalcores manufactured using continuous processes are extruded foams. Thesefoams possess good insulating properties, but generally poor structuralproperties. Therefore, there remains a long-felt and unmet need todevelop a cost-effective manufacturing technique to produce structuralinsulating cores in a one-step, continuous process. The continuousprocess would thus replace the existing, cumbersome, and expensivemulti-step processes.

Additionally, in conventional foamed products, as well as many otherextrusion processes (foaming and non-foaming), controlling thetemperature of the melt is a result-effective variable. For example,olefin-based polymers can be foamed only within limited temperatureranges. If the polymer melt enters and/or exits the die outside of theoptimum temperature range, the resulting product will be of lesserquality. In short, in many melt-flow processes (and especially thoseinvolving foaming of a polymer melt) the temperature must becontrollable within a given range to yield products having uniformphysical characteristics. The apparatus described herein allows forprecise temperature control of a flowing melt, in both foaming andnon-foaming processes.

SUMMARY OF THE INVENTION

The present invention provides an innovative manufacturing process forthe continuous fabrication of articles of manufacture, such aslightweight foamed panels of varying density, structural insulatingcomposite cores, and the like. The invention also encompasses theproducts produced using the process. The proposed manufacturing processutilizes an extruder to produce a panel or other cross-sectional designin a continuous manner, with the panel having varying foam densitiesand/or varying foam densities and solid sections throughout itsthickness. The variability of the foam densities (and/or the positioningof the solid sections) within the panel can be controlled using themethod. The controlled density variations within the panel can rangebetween highly foamed to solid (non-foamed) polymers. The controlleddensity variations in the panel result in a product with excellentstructural and insulating properties while remaining extremelylightweight. Exemplary cross-sectional configurations of the productsaccording to the present invention are shown in FIGS. 2A, 2B, 2C, 2D.

The present invention allows the manufacture of articles, such ascomposite panels, without the need for any secondary processing.Stiffening sections within the article are preferably non-foamed andhave a density at or near the density of the unprocessed polymer resinused. The stiffening sections are preferably separated with low-densityfoamed sections. (Because of their foamed nature, these sectionsnaturally act as thermal and sound insulators.) The low-density foamedsections also add still further stiffness to the article, especially ifthe article is a large panel or structural core. The density of thefoamed sections can be further customized to optimize for desired enduses, such as for thermal insulation, sound-dampening, and/or shockabsorption. The invention affords significant cost benefits toconventional fabrication techniques and yields articles havingcomparable or improved structural and insulating properties as comparedto conventional products.

The process can be used to extrude single-ply articles, or to co-extrudemulti ply articles, such as laminates, co-axial co-extrusions,co-extrusions encompassing a reinforcing matrix, etc. The resin can beextruded in any shape or profile, without limitation, including (but notlimited to) sheet form, circular, hollow, square, or any other geometriccross-section.

A first version of the invention is directed to an apparatus forcontrolling the temperature of a polymer melt or other flowing material.The apparatus comprises a row of baffles. The row itself comprises aplurality of baffles. Each baffle defines a longitudinal axis that ispreferably parallel to the longitudinal axis of another baffle in therow (although this is not required; see FIG. 20). Each baffle includesan upstream portion and a downstream portion. In one version of theinvention, the downstream portion of each baffle has a width that iswider than the upstream portion. In another version of the invention,each baffle has a diamond-shaped cross-section. In all versions of theinvention, the downstream portion of each baffle is convex or pointedand each baffle has a non-cylindrical cross-section perpendicular to itslongitudinal axis. A closed-circuit temperature-control conduit isdefined or disposed within at least one of the baffles. As used herein,the term “closed-circuit” means that the temperature-control mediumdisposed within the conduit does not come into contact with the meltflowing through the apparatus.

The temperature-control conduit can take a number of different forms.For example, the conduit can be a void defined within the baffle. Thevoid is configured to allow a temperature-control medium to flow withinthe conduit. In this fashion, a thermostatically-controlled liquidmedium (such as process water or mineral oil) can be circulated throughthe voids within each baffle. Alternatively, the temperature-controlconduit can be a solid, thermal-control device disposed within thebaffle, such as a thermostatically-controlled metallic or ceramicheating element. The apparatus may be configured so that there is aconduit defined or disposed within each baffle, or only in selectedbaffles. The temperature of each baffle can be controlled independentlyfrom any of the other baffles.

The apparatus according to the present invention may optionally comprisea die lip or body dimensioned and configured to yield an extrudatehaving a predetermined profile, such as a planar profile. Alternatively,the apparatus may be situated as an intermediate device in a modulararrangement of devices. When the present apparatus is placed at anintermediate position within the flow path, and a predetermined profileis desired, a final die that yields the desired profile is placeddownstream from the apparatus according to the present invention.

Another version of the invention is directed to a corresponding methodfor manufacturing variable-density polymeric articles. Thus, the methodcomprises passing a polymer melt through an apparatus comprising a rowof baffles as described in the immediately preceding paragraphs. Againthere is a temperature-control conduit defined or disposed within atleast one of the baffles to control the baffle's temperature. Thetemperature of the baffles is varied via the temperature-control conduitas the polymer melt passes through the apparatus. The temperaturevariations cause the density of the polymer melt passing proximate tothe baffle to be altered as compared to density of the polymer meltpassing distal to the baffle, thereby yielding a polymeric articlehaving variable density. In the preferred embodiment, the temperature ofthe baffles is regulated to be colder than the bulk temperature of thepolymer melt. This causes the density of the polymer melt that touchesthe baffles or passes proximate to the baffles to be of greater densitythan those portions of the melt that pass more distant from thetemperature-controlled baffles.

The resulting polymeric articles are also within the scope of theinvention. Thus the invention encompasses a variable-density polymericarticle comprising a continuous, monolithic, polymeric body, withoutjoints or seams, and having defined therein areas of higher densitydisposed adjacent to areas of lower density. The polymeric body can takeany desired cross-section shape. For example, the polymeric body can beplanar, in which case the areas of higher density may be disposedsubstantially perpendicular to the planar profile, substantiallyparallel to the planar profile, or at non-perpendicular, non-parallelangles to the planar profile (or any combination thereof).

It is therefore an object of the present invention to provide acontinuous method and device for manufacturing lightweight compositearticles at a significantly reduced cost.

It is a further object of the present invention to provide a continuousmethod and device for manufacturing a lightweight composite panel thatprovides the ability to control the density of the panel at any pointthroughout the cross-section of a profile during the fabricationprocess. These varying densities are essential to obtain the desiredstructural and insulating properties within a suitably lightweightpanel. It is a further object of the present invention to provide acontinuous manufacturing process that yields articles of manufacturehaving varying density, but which does not require secondarymanufacturing steps.

It is a further object of the present invention to provide a low-cost,continuous method (and a corresponding device) for manufacturinglightweight composite articles, such as panels, wherein the methodrequires no added secondary manufacturing processes. This eliminates theneed for adhesives or other chemicals currently required duringsecondary processing. Further, a method for manufacturing lightweightcomposite panels, structural insulating cores, and the like, in aone-step continuous process offers enormous cost advantages.

It is a further object of the invention to provide a versatile methodand device for manufacturing lightweight composite articles. Theversatility of this manufacturing technique allows easy adaptability tomore sophisticated products, as well as for more complex applicationswhere a variety of properties are required from the same product at thelowest cost possible.

It is a further object of the present invention to provide a continuousmethod and device for manufacturing a lightweight composite articlehaving a wide applicability in sandwich composites for many industrialsectors and in particular for the transportation and constructionsectors. The transportation sector devotes significant efforts towarddeveloping lighter, more cost-effective products. Further, in theconstruction sector, polymers and composites are experiencing greateracceptance and use. These trends illustrate a driving market force forlightweight structural and insulating panels that can be manufactured ata reduced cost.

It is a further object of the present invention to provide a method anddevice for manufacturing lightweight composite cores for wide softfoams, structural foams, foam-filled structural cores, andmulti-processed cores.

It is a further object of the present invention to provide a lightweightcomposite panel made from the same polymeric material, thereby providinga 100% recyclable product. Panels constructed using such cores is alsoencompassed within the present invention.

It is yet a further object of the present invention to provide acontinuous method and corresponding device for manufacturing lightweightcomposite articles wherein both the process and the resulting articleshave advantages over conventional manufacturing methods and cores. Theadvantages of the present invention include: better quantification ofthe key process physics that impact the density of foamed products as itapplies to continuous article manufacturing and the ability to gaininsight into process physics that induce varying foaming densities onthe extrudate's cross-section. The process of the present invention iscontinuous, thus yielding considerable cost savings as compared tobatch-type manufacturing methods.

The invention has many utilities. Primarily, the temperature-controlledapparatus can be used to make polymeric panels that can be used asstructural members, as thermal insulation panels, as acoustic insulationpanels, and the like. The temperature-controlled apparatus can be usedas an intercooler to control the temperature of a polymer melt streamduring processing. The apparatus can also be used to control thetemperature of any other materials passed through the apparatus When theapparatus includes several off-set rows of baffles, the apparatus canalso be used as both a mixer head and an intercooler.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a conventional, prior art compositesandwich panel showing opposing skins 40 and core 42.

FIGS. 2A, 2B, 2C, and 2D show exemplary cross-sections ofvariable-density cores manufactured according to the present inventionhaving foamed areas of lower density 44 and foamed or non-foamed areashaving increased density 46. FIG. 2A depicts alternating layers offoamed and non-foamed areas perpendicular to the surface of the core.FIG. 2B depicts diagonal layers of non-foamed areas. FIG. 2C depicts ahoneycomb-like arrangement of foamed and non-foamed areas. FIG. 2Ddepicts alternating layers of foamed and non-foamed areas parallel tothe surface of the core.

FIG. 3 is a schematic of an apparatus according to the present inventionshowing the location of the apparatus 10 disposed between the extruder(not shown) and a die 50. The arrow depicts the direction of materialflow.

FIG. 4 is a cross-sectional view of the inventive apparatus 10 showingcooling lines or temperature-control conduits 60 embedded withinteardrop-shaped baffles 12. The arrow depicts the direction of materialflow.

FIG. 5 illustrates the flow streamlines around the baffles 12.

FIG. 6 is a schematic of a sheeting die apparatus 50 according to thepresent invention having internal cooling coils 60 disposed withinteardrop-shaped baffles 12.

FIG. 7 illustrates temperature contours of flow melt through theapparatus (with gap distance 13) with cylindrical baffles 12 and gapspace 13 (cooling conduits removed for clarity). The resulting foamedareas 44 and non-foamed areas 46 of the melt as it exits the apparatusare also depicted.

FIG. 8 is a graph illustrating the temperature profile of the meltpassed over the apparatus at different gap distances between coolinglines.

FIG. 9 is a graph illustrating the pressure drop rate of the melt as itflows through the apparatus at different gap distances.

FIG. 10 is a front perspective view of an apparatus according to thepresent invention.

FIG. 11 is a rear perspective view of the apparatus of FIG. 10.

FIG. 12 is a side view of the apparatus of FIG. 10.

FIG. 13 is a front view of the apparatus of FIG. 10.

FIG. 14 is a side view of two rows of baffles from the apparatusillustrated in FIG. 10 with cooling conduits 60 shown only in the bottomrow of baffles for clarity.

FIG. 15 is a side view of two rows of baffles from an alternativeversion of the present invention with cooling conduits 60 shown only inthe bottom row of baffles for clarity.

FIG. 16 illustrates alternative baffle shapes with cooling conduits 60shown only in the bottom row of baffles for clarity.

FIG. 17 is a longitudinal cross-sectional view of another version of theinvention having baffles with a roughly diamond-shaped cross-section.Here, each row of baffles is parallel with every other row of baffles.

FIG. 18 is a perspective view of another version of the inventionwherein the baffles 12 have a roughly diamond-shaped cross-section, butthe two right-hand rows of baffles are rotated 90 degrees with respectto the two left-hand rows of baffles.

FIG. 19 is another perspective view of FIG. 18 more clearly depictingthe conduits 60 and the modular construction of the apparatus.

FIG. 20 is a perspective view of another version of the inventionwherein the baffles within a single row are not parallel to one another.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a method and an apparatus for thecontinuous, one-step manufacture of lightweight, low-cost, articles ofmanufacture having a controlled density. The articles so produced arealso included within the present invention. The method and apparatus ofthe present invention better quantifies the process physics that impactthe density of foamed products as it applies to continuousmanufacturing, as well as the quality of both foamed and non-foamedproducts. The invention thus providing optimum processing conditions toproduce foamed resins with varying densities on an extruder and toensure the quality of non-foamed products.

The apparatus according to the present invention will function using anypolymer resin that can be extruded, as well as any other product thatcan be passed through the apparatus. The principal utility of theinvention, though, is in the extrusion of polymeric articles. Thus, anon-limiting list of polymers that can be used in the present inventioninclude styrenic resins, olefinic resins, acrylates, methacrylates,acrylimides, methacrylimides, carbonates, poly(arylene) oxides,polyvinyl alcohols, co-polymers of any of these (e.g., ABS), and thelike. Elastomeric polymers and rubbers (natural and synthetic) may alsobe used in the present invention. Explicitly included within the list ofpolymers that can be used in the invention are polystyrenes (PS)(preferred), polyethylenes (PE), polypropylenes (PP), polyvinylchlorides(PVC), polyvinylidene chlorides (PVdC), polyurethanes (PU),polyphenylene oxides (PPO), polycarbonates (PC), polyvinylalcohols(PVOH) and polymethacrylimides (PMI).

As noted above, the method of the present invention brings together theproper combination of material, foaming agent ratios, and equipment forthe continuous manufacture of lightweight composite articles, such aspanels and cores.

Numerous researchers have studied the effect of processing parameters onfoaming density [13-21]. The simplest studies vary the foaming agentconcentration, the melt temperature, or the pressure differential duringfoaming. However, varying these parameters requires multiple steps inthe manufacturing process. Thus, in the present invention, varying thefoaming agent concentration is not a critical, or even a useful,parameter. Therefore, in one version, the present invention comprisespremixing foaming agent with polymer resin pellets [25] by adding asuitable amount of foaming agent per unit mass of the resin. The resinand the foaming agent are then thoroughly mixed, for example, by meansof a mixing screw. Then, approximately 0.2% to 5.0% by weight of achemical or physical foaming agent is added. In this manner, an extrudedproduct with a controlled foam density across its cross-section isproduced as depicted in FIGS. 2A, 2B, 2C, and 2D. In these figures, theextruded product is depicted as a planar sheet. Thus, the areas ofincreased density may be disposed perpendicular to the surface of thesheet (FIG. 2A), parallel to the surface of the sheet (FIG. 2D), at anyother angle with respect to the surface of the sheet (FIG. 2B), or inany other desired pattern, such as a honeycomb-like pattern (FIG. 2C).

The invention will function using either chemical or physical foaming orblowing agents, which are equally preferred. Chemical foaming, can beaccomplished using endothermic or exothermic foaming agents. Physicalfoaming using any type of physical foaming or blowing agent (e.g.,carbon dioxide, nitrogen, alkanes, halogenated alkanes, otherhydrocarbon based blowing agents, etc.) may also be used in the presentinvention. The apparatus can also be used in non-foaming applicationsfor mixing, for temperature control, or both.

By isolating the effects of die temperature and pressure, Park et al.[19] show that the most important factor on nucleation rate, and thusfoam density, is the pressure drop rate. This is because the pressuredifference that induces foaming is the real source of thermodynamicinstability. If the nucleation time is kept constant, this thermodynamicinstability is larger as the pressure drop rate increases. Therefore,even though it is well settled that the pressure drop rate is the mostinfluential variable affecting foam densities, temperature and pressurecombine to have a greater effect on foam quality [13-14, 16-21].Temperature and pressure also have a large influence on secondaryvariables that affect nucleation and foam density, such as meltviscosity, solidification, coalescence and the diffusion of the foamingagent out of the extrudate. All of these variables must be consideredwhen developing a manufacturing technique that is able to vary foamdensities across the extrudate.

The present invention uses an internal apparatus to control and varytemperature (and thus pressure drop) on the melt to induce varyingfoaming densities on the extruded polymer or to maintain the melt withina desired and predetermined temperature range. The apparatus comprisesbaffles with a converging section followed by a diverging sectionthrough which the polymer melt can flow. One embodiment is a teardropcross-section as seen in FIGS. 3, 4, and 5 (other cross-sections,described hereinbelow, are also within the scope of the invention).Referring specifically to FIGS. 3, 4, and 5, the inventive apparatus 10,comprises a row of baffles 12 (see FIG. 5). At least one of the baffles12 includes a temperature control conduit 60 disposed therein or passingtherethrough, as shown in FIG. 4. The temperature control conduitfunctions to regulate the temperature of the material that comes intocontact with the baffles (or passes in close proximity to the baffles.The temperature control conduit is a closed circuit, meaning that thematerial disposed within the conduit 60 (either a circulating fluid or aheating/cooling element) is not in direct contact with the materialpassed through the apparatus. In the embodiment depicted in FIGS. 3 and4, the apparatus 10 is placed in the flow path between an extruder (notshown) and a second die 50 as shown in FIG. 3. The die 50 can bedimensioned and configured to yield any desired cross-sectional profileto the extruded product (e.g., circular, regular or irregular polygons,planar, etc.) As the melt flows between the teardrop baffles, the meltundergoes elongational flow mixing. The melt that contacts the baffles12 is either heated or cooled (depending upon the temperature of thebaffles relative to the temperature of the bulk melt as the melt impactsand passes over the baffles).

To use the apparatus 10 as a temperature control device (i.e., anintercooler), a temperature control conduit 60 is defined or disposed ineach baffle 12. This can be done, for example, by definingtemperature-control conduits 60 through the length of the baffles, asseen in FIG. 4. (See also FIGS. 17-20, described below.) These conduitsare connected at each end to a source of temperature-controlled liquidor gas (not shown). As the liquid/gas flows inside the conduits, itchanges and controls the temperature of the baffles 12, which are incontact with the polymer melt, thereby resulting in a user-variable anduser-controllable temperature field downstream from the baffles.Alternatively, the temperature-control conduits may be solid heating orcooling elements (e.g., metallic or ceramic elements) embedded within orotherwise incorporated onto or into the baffles 12.

The converging sections of the baffles also act as restrictors, defininga converging flow path. The converging sections of the baffles act toinfluence the temperature of the material as it exits the gap betweenthe baffles toward the diverging section of die 50 (see FIG. 3). Varyingthe temperature of the fluid, gas, or heating/cooling element inside thetemperature-control conduit 60, as well as varying the gap betweenbaffles, has a direct influence on the temperature and pressurevariations of the melt stream. Because foam density is affected bypressure and temperature, the apparatus described herein here is able tovary the density within defined regions of the extrudate. Thus, byjudiciously selecting the temperature of the baffles and by selectingsuitable gap sizes between the baffles, cores of variable density can becreated at will.

The temperature of the circulating fluid passing through the conduits 60is controlled by an external circuit that preferably includes aheater/refrigerator unit as well as suitable thermostat elements. Ifdesired, the temperature within each baffle can be selectivelyadjustable independently of the other baffles. If this is desired, eachbaffle includes its own external conduit and associated temperaturecontrol elements to maintain each baffle at a desired temperature.

The preferred version of the invention utilizes the teardrop shape ofthe baffles to maximize heat conduction and convection from the heatingelement or liquid circulating inside the baffles toward the polymer meltflowing around the baffles. Thus, it is important to ascertain theappropriate dimensioning of the equipment before large-scalemanufacturing commences. In short, the apparatus 10 must be “dialed in”to establish the appropriate values for melt pressure drop, temperature,the dimensions of the baffles 12, the spacing between the baffles, thenumber of baffles, the orientation of the baffles, the temperature ofthe temperature control conduit 60, the liquid circulation rate (if aliquid temperature control mechanism is used in the conduit 60), and thepolymer melt flow rate to achieve the desired variability in the densityof the core. These parameters are established empirically.

The apparatus of the present invention can be inserted within the flowpath of any extruder capable of extruding polymeric resins, withoutlimitation. The device is preferably located at the end of the flow path(e.g. see FIG. 6) or near the end of the flow path (e.g., see FIG. 3)and may comprise multiple sections (e.g. see FIGS. 12 and 19) to permitcustomized cores or other desired profiles to be produced.

If the temperature control device is placed immediately before the flowpath exit, the sudden pressure drop will induce foaming downstream fromthe baffles. FIG. 5 shows the computed streamlines as the material flowsaround the teardrop baffles 12. Here the expected foaming area insidethe melt, downstream from the baffles, is also shown. In the presentinvention, heating or cooling is taking place at the same time the meltis experiencing a large pressure drop as it exits the die.

The present invention differs significantly from earlier processes (suchas the Celuka process [24]) by utilizing internal cooling (or heating)to induce controlled varying densities within the cross-section of theextrudate. The present invention therefore includes an apparatus 10 asshown in FIGS. 3 and 4. In these two figures, the apparatus is depictedas having a circular cross-section, which is generally preferred. Thatbeing said, other, non-circular cross-sections for the apparatus 10 as awhole are within the scope of the invention. The present invention alsoincludes a die 50 as shown in FIG. 6 to extrude a foamed sheet, as anextruded sheet can be directly used as the core material in a compositesandwich panel.

Specifically referring to FIG. 6, this figure depicts another embodimentof the present invention in which the baffles 12, with their associatedtemperature-control conduits 60, are integrated within the final die 50that gives the extrudate the desired final profile. Here, the die 50 isdepicted as a sheet die, and the baffles 12 are placed just before thedie exit.

To improve heat transfer, the temperature control conduit 60 inside eachbaffle is preferably as large as possible. However, a careful balancebetween heat transfer and structural integrity must be maintained toavoid failure during processing. Finite element structural analysis(FEA) combined with non-isothermal flow analysis can be employed tobalance these properties (i.e., to balance the structural integrity ofeach baffle 12 versus the void volume of the temperature-control conduit60 within each baffle).

The apparatus of the present invention is preferably of a modular designto allow the addition of various internal cooling devices. See FIG. 19.The apparatus is preferably dimensioned and configured to allow changingthe axial location of the apparatus with respect to the die exit.

An exploratory non-isothermal flow simulation was done to estimate thetemperature and pressure differentials as the material flows by thetemperature-regulated baffles. The exemplary flow simulation, performedin two dimensions, assumes the baffles have a circular cross-section asshown in FIG. 7 (a cylindrical cross-section simplifies thecalculation). For simulation purposes, the walls of the baffles wereassumed to be at 343 K, and the melt at 513 K. Material properties for atypical PS resin were selected. The cross-section was set at 46 mm inheight, the diameter of each baffle 12 at 10 mm, and the gap 13 betweenbaffles at 3 mm. (This is a simplified analysis for sake ofillustration. The baffle shape and the actual processing parametersemployed during any given manufacturing run will vary considerably.) Theresults, depicted on the right-hand portion of FIG. 7 is an extrudatehaving lower-density areas 44 and higher-density areas 46.

FIG. 8 shows the temperature contours obtained from this flowsimulation. The contours show a layered temperature that varies throughthe melt. The temperature variations have an effect on the foamingdensity of the extrudate. The temperature variations throughout themelt, as shown in the graph of FIG. 8, can be purposefully altered byadjusting the distance of the gap between the baffles. Thus, forexample, FIG. 8 depicts the temperature of the melt as a function of thedistance of the melt from the baffles when the gap between the bafflesis 3 mm vs. 15 mm. (FIG. 8 assumes a flow speed of 10 mm/s.) As shown inFIG. 8, temperature variations of up to 60 degrees are observed at flowspeeds of 10 mm/s and a 3 mm gap. The temperature variations thus giverise to corresponding density variations once the extrudate hardens intoits final form.

The other variable controlled by the geometry of the apparatus is themelt pressure. The influence of the gap distance between baffles on thepressure drop rate of the melt is shown in FIG. 9, which is a graph ofmelt pressure with respect to the axial distance for two gap geometries(3 and 15 mm). Here, the effect of the gap size is observed to vary thepressure drop rate more than 60-fold. The results of the exploratoryflow simulation shown in FIGS. 7-9 demonstrate that the presentinvention yields a polymeric article of manufacture with varyingdensities.

Conversely the influence of the gap distance between baffles, bafflegeometry, and conduit design can be such to control the melt streamtemperatures. These temperatures can be so fashioned as to inducetemperature variations or to thoroughly homogenize the temperaturevariation of the melt stream.

Fractional factorial information is preferably used to facilitate thecorrelation of the many processing parameters and design variables withthe desired measurements. The results obtained can be further correlatedback to the simulated temperature and pressure results to establish aconnection between simulated temperature and pressure variations withexperimental results to facilitate design, setup and scale-up of thepresent invention. Non-Newtonian non-isothermal flow simulations arepreferably performed to guide the design of the processing equipment andthe temperature control conduits.

The apparatus 10 according to the present invention is illustrated ingreater detail in FIGS. 10-20. (For clarity, the temperature controlconduits 60 are omitted in FIGS. 10-13.

The apparatus 10 includes a series of baffles 12 arranged in rows. Inthe embodiment illustrated in FIGS. 10-13, each baffle 12 isteardrop-shaped and includes a large rounded head portion 14 on one endand a small tail portion 16 on the opposing end. The head portion 14 andtail portion 16 are interconnected by a gradually diverging portion 18.In the versions depicted in FIGS. 10-19, each baffle 12 defines alongitudinal axis 15 that is parallel to the longitudinal axis(es) ofthe other baffle(s) in the same row. In FIG. 20, the longitudinal axesof the various baffles 12 within the row are not parallel.

The head portion 14 of each of the baffles 12 shown in FIGS. 10-16 issemi-cylindrical in shape. The diameter of the head portion 14 isdimensioned to be the same as the maximum width of the baffle 12 so thata smooth transition occurs between the diverging portion 18 and the headportion 14 of each baffle. The large diameter head portion facilitatessmooth flow of the melt stream through the apparatus 10 with reduceddead zones (i.e., reduced stagnation).

Preferably, the baffles 12 are positioned within the apparatus so thatthey are parallel to and slightly spaced from the other baffles withinthe same row. See FIGS. 14 and 15. Although this is not required, asillustrated in FIG. 20. The baffles are oriented relative to thematerial flow (illustrated by arrows in FIGS. 10 and 12) such that thematerial passes between the baffles from the tail portion 16 to the headportion 14. In this orientation, the tail portion 16 and divergingportion 18 of each baffle form an upstream portion 17, while the headportion 14 forms a downstream portion 19, as shown in FIG. 12. With theabove-described orientation of the baffles 12, it can be seen that thediverging portions 18 of adjacent baffles 12 form a converging pathwayat a converging angle (FIG. 12, top) through which the melt stream willflow. This converging pathway provides compressive forces on the meltstream, resulting in elongation and dispersion of the melt stream. Itshould be appreciated that there are other orientations of the bafflesthat could also form the desired converging pathway. For example,instead of the illustrated straight walls of the diverging portion 18,the converging pathway could instead be formed by curved walls thatachieve the same elongational results.

The precise dimensions of the apparatus 10 will vary depending on thematerials to be processed and the cross-sectional area of the flow path.For example, the length, width, and number of baffles can be chosen tomeet specific needs. Furthermore, the converging angle and the gap(i.e., at the narrowest point) between adjacent baffles can further bevaried to achieve different compressive and elongation forces as well ascontrol temperatures.

In the embodiment shown in FIG. 12, the converging angle is betweenabout 14 degrees and about 100 degrees. Preferably, the converging angleis between about 20 degrees and about 80 degrees, and more preferablythe converging angle is between about 40 degrees and about 70 degrees.In addition, the ratio of the baffle gap to the baffle width (i.e., thegap:width ratio) is preferably between about 1:7 and about 2:5.

Adjacent rows of the baffles 12 may be transversely oriented (e.g.,rotated) relative to each other in order to facilitate distributivemixing and temperature homogenization. See FIGS. 13 and 19. Morespecifically, the illustrated longitudinal axes 15 of the baffles 12 ofone row are angled about 90 degrees relative to the longitudinal axes 15of the adjacent rows, as shown in FIG. 11. With this design, it can beseen that the baffles 12 of alternating rows will be parallel to eachother. In addition to being parallel, the baffles 12 of alternating rowsmay also be staggered slightly to further promote distributive mixingand temperature homogenization. It should be appreciated that theangular change of the baffles 12 could be less than 90 degrees, such as45 degrees, thereby promoting a more gradual twisting of the melt streamas it passes through the apparatus.

In addition to promoting compression and elongation of the melt stream,the teardrop-shaped baffles 12 also reduce the amount of dead zoneswithin the apparatus 10. Static mixers and intercoolers typicallyinclude dead zones within sharp corners, and particularly in transitionregions with concave portions that face downstream. The baffles 12alleviate this problem by providing downstream portions 19 that aregenerally convex or pointed in shape (e.g., the rounded head portions14). The rounded head portions 14 promote flow around the downstream endof the baffles 12 to reduce the amount of dead zones within theapparatus 10.

The apparatus 20 illustrated in FIG. 14 further reduces the amount ofdead zones by overlapping the baffles 22 of one row with the baffles 22of the adjacent row. By doing this, the amount of downstream dead zonesare further reduced. In the illustrated embodiment, the tail portion 24of one row of baffles 22 is positioned approximately at the point ofmaximum compression 26 of the previous row of baffles 22. Suchpositioning of the baffles forces the melt stream to pass immediatelyfrom the zone of maximum compression of one row of baffles into thecompression zone of the next row of baffles. This further enhances thetemperature homogenization and mixing of the materials. FIG. 15 depictsthe analogous embodiment wherein the baffles 12 of one row do notoverlap with the baffles of the adjacent row.

FIG. 16 illustrates alternative baffle shapes, and specificallyillustrates alternative head portion shapes. For example, the headportion 30 could have a semi-elliptical or semi-oval shape, whichprovides a more gradual downstream transition zone and is believed tofurther reduce material stagnation. Alternatively, the head portion 32could be pointed, which provides a constant downstream expanding angle.

FIGS. 17-19 depict another version of the apparatus 10 wherein thebaffles are not tear drop-shaped but roughly diamond-shaped oroval-shaped. Here, the baffles have a pointed tail region and a pointedhead region. As depicted in FIG. 17, the pointed head and tail regionsof each baffle are symmetrical in the plane perpendicular to the flowpath (the direction of material flow)—that is, each baffle has planarsymmetry through a vertical plane passed through the center of eachbaffle as shown in FIG. 17. Because the head and tail regions of thebaffles as shown in FIGS. 17-20 are symmetrical, this version of theapparatus does not have a directionality. A melt passed fromleft-to-right in the version depicted in FIG. 17 experiences the sameforces as a melt passed from right-to-left. (The same is not the casefor the version of the invention depicted in FIG. 4, for example,wherein the baffles lack such planar symmetry. Note, however, that thisfact does not preclude running material through the apparatus as shownin FIGS. 4, 5, 7, and 12 in the direction opposite to the arrow ifdesired.) As shown in FIGS. 17-19, in this version a temperature-controlconduit 60 is present in every baffle. In FIG. 17 each row of baffles 12is parallel to the row before it; the rows are not rotated. In FIGS. 18and 19, however, some of the rows are rotated with respect to oneanother.

FIG. 19 shows how the apparatus of the present invention can befabricated in a modular fashion. Here, each row of baffles is disposedwithin its own separate module. The order and orientation of the modulescan be controlled by the user to vary the overall geometry of thecollection of modules.

FIG. 20 depicts yet another version of the invention wherein thelongitudinal axes of the baffles 12 are not parallel. As shown in FIG.20, baffle 12′ is disposed in a non-parallel relationship to baffle 12.

The foregoing description of the present invention has been presentedfor purposes of illustration and description. Furthermore, thedescription is not intended to limit the invention to the form disclosedherein. Consequently, variations and modifications commensurate with theabove teachings, and the skill or knowledge of the relevant art, arewithin the scope of the present invention. The embodiments describedherein are further intended to explain best modes known for practicingthe invention and to enable others skilled in the art to utilize theinvention in such, or other, embodiments and with various modificationsrequired by the particular applications or uses of the presentinvention. It is intended that the appended claims be construed toinclude alternative embodiments to the extent permitted by the priorart.

REFERENCES

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1. An apparatus comprising: a row of baffles, the row comprising aplurality of baffles wherein each baffle defines a longitudinal axis,wherein each baffle includes an upstream portion and a downstreamportion, wherein the downstream portion of each baffle is convex orpointed, wherein each baffle has a non-cylindrical cross-sectionperpendicular to its longitudinal axis, and further wherein the bafflesof each row define a converging flow path for materials passing throughthe apparatus; and a closed-circuit, temperature-control conduit definedor disposed within at least one of the baffles.
 2. The apparatus ofclaim 1, wherein the longitudinal axis of each baffle in the row isparallel to the longitudinal axis of another baffle in the row, andwherein the downstream portion of each baffle has a width wider than theupstream portion, and wherein the downstream portion of each baffle hasa convex surface.
 3. The apparatus of claim 1, wherein thetemperature-control conduit is a void defined within the baffle, whereinthe void is configured to allow a temperature-control medium to flowwithin the conduit.
 4. The apparatus of claim 1, wherein thetemperature-control conduit is a solid thermal-control device disposedwithin the baffle.
 5. The apparatus of claim 1, comprising a conduitdefined or disposed within each baffle.
 6. The apparatus of claim 1,wherein the downstream portion of each baffle has a convex shape.
 7. Theapparatus of claim 1, comprising a plurality of rows of baffles, whereinthe longitudinal axes of the baffles in one row are transverse to thelongitudinal axes of the baffles in an adjacent row.
 8. The apparatus ofclaim 7, wherein the axes of one row of baffles are rotated about 90degrees relative to the axes of the baffles in an adjacent row.
 9. Theapparatus of claim 7, wherein the axes of the baffles in one row areparallel to the axes of the baffles in a non-adjacent row.
 10. Theapparatus of claim 7, wherein the baffles in one row overlap with thebaffles of an adjacent row.
 11. The apparatus of claim 1, wherein thebaffles are substantially teardrop in shape.
 12. The apparatus of claim1, wherein the downstream portion of at least one baffle issemi-cylindrical in shape.
 13. The apparatus of claim 1, wherein thedownstream portion of at least one baffle is semi-elliptical in shape.14. The apparatus of claim 1, wherein the upstream portion and thedownstream portion of each baffle are pointed.
 15. The apparatus ofclaim 14, wherein the upstream portion and the downstream portion ofeach baffle are symmetrical about a plane perpendicular to direction offlow of material through the apparatus.
 16. The apparatus of claims 1,further comprising a die lip dimensioned and configured to yield anextrudate having a predetermined profile.
 17. The extrusion die of claim16, wherein the die lip is dimensioned and configured to yield anextrudate having planar profile.
 18. A method for manufacturingvariable-density polymeric articles, the method comprising: (a) passinga polymer melt through an apparatus comprising a row of baffles, the rowcomprising a plurality of baffles wherein each baffle defines alongitudinal axis, wherein each baffle includes an upstream portion anda downstream portion, wherein the downstream portion of each baffle isconvex or pointed, wherein each baffle has a non-cylindricalcross-section perpendicular to its longitudinal axis, and furtherwherein the baffles of each row define a converging flow path formaterials passing through the apparatus; and a closed-circuit,temperature-control conduit defined or disposed within at least one ofthe baffles to control the baffle's temperature; and (b) varying thetemperature of the baffle via the temperature-control conduit as thepolymer melt passes through the apparatus, wherein the temperature isvaried to cause density of the polymer melt passing proximate to thebaffle to be altered as compared to density of the polymer melt passingdistal to the baffle, thereby yielding a polymeric article havingvariable density.
 19. The method of claim 18, wherein in step (a) thelongitudinal axis of each baffle in the row is parallel to thelongitudinal axis of another baffle in the row, and wherein thedownstream portion of each baffle has a width wider than the upstreamportion.
 20. The method of claim 18, wherein step (b) comprises varyingthe temperature of the baffle such that the density of the polymer meltpassing proximate to the baffle is increased as compared to the densityof the polymer melt passing distal to the baffle.
 21. The method ofclaim 18, wherein in step (a), the apparatus is dimensioned andconfigured to yield a polymeric article having a predetermined profile.22. The method of claim 21, wherein in step (a), the apparatus isdimensioned and configured to yield a polymeric article having a planarprofile.
 23. The method of claim 18, further comprising, after step (b),(c) and then passing the polymer melt through a die dimensioned andconfigured to yield a polymeric article having a predetermined profile.24. The method of claim 18, further comprising, after step (b), (c) andthen passing the polymer melt through a die dimensioned and configuredto yield a polymeric article having a planar profile.
 25. The method ofclaim 18, wherein step (b) comprises varying the temperature of thebaffle to yield areas of increased density within the polymer article,and further wherein the areas of increased density are disposedsubstantially perpendicular to the planar profile, substantiallyparallel to the planar profile, or at non-perpendicular, non-parallelangles to the planar profile.
 26. The method of claim 18, furthercomprising: (c) co-extruding at least one additional polymer melt toyield a composite, variable-density polymeric article.
 27. Avariable-density polymeric article comprising: a continuous, monolithic,polymeric body, without joints or seams, and having defined thereinareas of higher density disposed adjacent to areas of lower density. 28.A variable-density polymeric article of claim 27, produced by: (a)passing a polymer melt through a die comprising a row of baffles, therow comprising a plurality of baffles, each baffle defining alongitudinal axis that is parallel to the longitudinal axis of anotherbaffle in the row, wherein each baffle includes an upstream portion, anda downstream portion having a width wider than the upstream portion; anda closed-circuit temperature-control conduit defined or disposed withinat least one of the baffles to control the baffle's temperature; and (b)varying the temperature of the baffle via the temperature-controlconduit as the polymer melt passes through the die, wherein thetemperature is varied to cause density of the polymer melt passingproximate to the baffle to be altered as compared to density of thepolymer melt passing distal to the baffle, thereby yielding a polymericarticle having variable density.